This invention relates to a code signal blanking apparatus, and more particularly, but not by way of limitation, an apparatus for blanking a code signal superposed on a television signal during a vertical blanking period in a pay-TV system.
A so-called wireless pay-TV system, to which the present invention is applicable, has been developed to eliminate annoying commercial messages indispensable to ordinary commercial broadcasting and possible degradation of broadcasting quality due to various restrictions inherent to the commercial broadcasting. This wireless system is expected to be spread wide because expenses and time required for laying cables can be curtailed and there is no limitation in number of subscribers.
In general, in the wireless pay-TV system, a television signal and an aural signal are encoded so that the system may be applied only to the subscribers of the system, excluding non-subscribers. More specifically, a transmitting station includes an encoder for encoding a video signal by randomly inverting or not inverting the video signal with reference to a gray level according to a predetermined procedure. The encoder has a key code for encoding. The video signal is encoded based on the key code. At the same time, data for decoding prepared based on the same key code is added to the television signal as a code signal in the form of a binary code. On the other hand, a decoder provided at a receiving unit receives the encoded television signal, reads the code signal for decoding to reduce it to the key code and decodes the encoded television signal into an ordinary television signal using the key code. As to the aural signal, similar operation is carried out.
The code signal as mentioned above is inserted during a vertical blanking period when the beam does not appear on a picture of a television set. However, sometimes, the insertion of the code signal possibly causes disturbance of a vertical signal or appearance of a binary code on the picture.